Kaposi's sarcoma-associated herpes virus (KSHV), an important human pathogen accounting for a Large percentage of virally-caused cancers worldwide, has evolved a variety of stratagems for evading host immune responses to establish a life-long persistent infection and for deregulating cell growth control to achieve oncogenesis. The goal of Project 1 is to better understand how the KSHV evades host's innate and adaptive immune controls and subsequently deregulates host's growth controls, with a specific focus on the KSHV K3 and K5 genes. Our preliminary study has shown (1) that both the K3 and K5 membrane E3 ubiquitin ligase proteins downregulate MHC class I and CDId molecules, and the K5 downregulates numerous immune modulatory proteins (interferon gamma receptor 1, B7-2, ICAM-1, tetherin etc.). We have also discovered (2) that the K3 or K5 expression apparently increases cell proliferation and induced tumorigenicity in nude mice, and (3) that the K5 gene whose promoter carries an active epigenetic mark during latency is readily expressed in Kaposi's Sarcoma tumors, as well as in Primary Effusion Lymphomas and multicastleman's diseases. (4) Furthermore, we have developed an infectious KSHV bacterial artificial chromosome (BAC16) to facilitate the efficient genetic manipulation ofthe viral genome. A main hypothesis of Project 1 is that the KSHV has evolved to carry the K3 and K5 genes with similar, yet distinct biochemical activities to ensure comprehensive protection from host immune effectors and to deregulate cell growth control. Despite previous extensive cell biology and biochemical studies, the detailed in vivo biological evidences of K3- and K5-mediated immune evasion in viral persistence and pathogenesis are still elusive. In this proposal, we will attempt to define in vivo roles ofthe K3 and K5 in vial persistence and oncogenesis. Specifically, we will test whether the loss of K3 and/or K5 genes from the KSHV genome affects the establishment of viral persistence in NOD/SCID IL2Ry-/- humanized mice and the induction of viral oncogenic transformation of primary embryonic mesenchymal stem cells (MSC) in cultures and nude mice. This proposal is highly innovative and its successful outcome should significantly impact our understanding of KSHV biology.